Why is there a buzzing in my rear end?

2001 Ford explorer sport. so for the longest time I've had this buzzing in the rear end. for an unrelated issue, I've had the transmission rebuilt last year. Even before then. I've still had the buzzing and I still have it now. I had the rear differential changed about a decade ago. The buzzing used to be off and on but now it's more constant. it only really happens when I press the gas. If I don't press the gas and I just let the car coast the sound automatically stops. recently when I had my oil changed I had to look the rear differential fluid. they said it was full and clear so I know it's there and it's not full of metal shavings. so if it's not the rear differential then what could it possibly be us making this buzzing sound.